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Between 25 and 54 percent of people are thought to crack their knuckles. Reasons include a nervous habit and seeking relief from tension. Men do this more often than women.

Fast facts on knuckle cracking

Here are some key points about knuckle cracking. More detail is in the main article.

Between 25 and 54 percent of people crack their knuckles.

Most knuckle crackers are male.

The cracking sound has been linked to the formation of bubbles in the synovium, but this may not be true.

Cracking knuckles does not appear to cause or worsen arthritis, but it can soften the grip and lead to soft tissue swelling.

Causes

Cracking the knuckles can provide a sense of relief and relaxation for those doing it.

The exact reason for the popping and snapping of joints is not known, but there are some theories.

Synovial fluid bubbles and cavitation

One common theory has been that when a finger or joint is extended, the pressure inside the finger is lowered and gases that are present, such as carbon dioxide, are released in the form of a bubble.

The bubble-collapse theory suggests that popping joints creates a vacuum that the gases then fill. Then, when joints are extended through pulling, there is a sudden and dramatic increase in surrounding pressure.

This causes a sudden partial or total collapse of these gas bubbles, and this collapse is heard as the popping sound.

This formation and collapse of bubbles is known as cavitation.

Cavity formation

Research has debunked the bubble collapse theory of joint popping, on the basis of visual evidence of what happens in the joint as it cracks.

By collecting real-time footage using magnetic resonance imaging (MRI), one study concluded that joint cracking is related to cavity formation rather than the collapse of gas bubbles.

The scientists studied ten metacarpophalangeal joints by inserting volunteers' fingers into a flexible tube that
could be used to apply traction to the joint.

They took images using MRI before and after traction was applied to the point of cracking, at 3.2 frames per second.

The results showed the rapid creation of a cavity in the joint at the point of joint separation, and this produced the sound. The cavity remained visible after the noise.

This suggests that the cracking of the joint itself may be sufficient to cause rapid cavitation and the popping sound.

There may be no need for gas bubbles for the sound to occur.

Tendons snapping over joints

Some doctors propose that the cracking sound made by joints is the result of tendons snapping into place over joints.

Joint cracking is often confused with the snapping sound made by stiff tendons or other bands of soft tissue sliding between muscles or over bony outcrops.

Tendons keep muscles attached to bones, while ligaments connect bones to other bones.

Doctors believe that tendons can make a popping noise when they quickly snap over a joint. Ligaments may make popping noises when they tighten while the joint is moving.

When a joint moves, the tendon changes position relative to the joint. After the tendon changes position,
there may be a sudden snap as it returns to its original location.

These noises are often heard in the knee and ankle joints when standing up from a seated position or when walking up or down the stairs.

Shoulder joint pop, crack, or click

If the shoulder cracks or clicks painlessly, soft tissue in the joint is likely to be the cause.

If there is pain with the noise, there may be damage to the surfaces of the joint, possibly due to a tear in the ring of cartilage that acts as a seal around the shoulder joint. This ring of cartilage is known as the labrum.

The labrum may snap over the other structures of the shoulder joint as the arm moves.

Sometimes the clicking may be due to the shoulder slipping in and out of joint. This is known as shoulder instability or subluxation.

Does cracking the knuckles cause arthritis?

Cracking the knuckles is not thought to be linked to arthritis.

The most common joints that people choose to crack are the knuckles.

They tend to do this in one of three ways:

bending them backward or forward

turning them sideways

pulling on the bones around the joint

Despite popular beliefs, several studies have concluded that cracking knuckles is unlikely to be linked to
arthritis.

Dr. Donald Unger researched his own knuckle-cracking, in response to complaints from his family. He cracked the knuckles of his left hand at least twice a day for 50 years, but not those of his right hand.

Unger did not develop arthritis in either hand, and there were no differences between the two hands.

He concluded that knuckle cracking was not linked to arthritis.

The Uniformed Services University of the Health Sciences looked at 215 people, 20 percent of whom popped their knuckles regularly.

Of the knuckle crackers, 18.1 percent had arthritis in their hands, compared with 21.5 percent of participants who did not crack their knuckles.

The study showed that the chances of having arthritis are around the same whether or not you crack your joints.

Other concerns

However, arthritis is not the only concern.

Inflammation and weak grip

Joint popping may not cause arthritis, but there may be risks. At least one study has linked chronic joint popping with inflammation and weakened grip in the hands.

In 1990, researchers investigated 300 participants over the age of 45 years, 74 of whom cracked their knuckles, while 226 did not.

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